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Geology and Mineralogy of Gemstones. David Turner R.Читать онлайн книгу.

Geology and Mineralogy of Gemstones - David Turner R.


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through the examination of the external form of a mineral specimen, which can be either an individual crystal or an aggregate of crystals that grew together. Terms used to describe individual crystals include platy, pyramidal, bladed, lamellar, acicular, tabular, or prismatic. Terms used to describe aggregates of crystals include fibrous, reniform, botryoidal, dendritic, radiating, concentric, massive, or stalactitic.

      Cleavage, parting, and fracture describe the ways in which minerals break under force. Cleavage is the occurrence of discrete planes of weakness in a mineral that correlate to weaknesses in the internal bonding and atomic structure of that mineral. Cleavage is often described as perfect (as in micas), good (as in epidote), imperfect (as in beryl), or indistinct (as in tourmaline). Cleavage planes will be straight and repeated in different orientations of a mineral as dictated by that mineral’s overall symmetry, and therefore can be described using crystallographic orientations and patterns (e.g., octahedral cleavage, as in fluorite).

      Hardness is a measure of a mineral’s resistance to scratching against another mineral and is related to its bonding characteristics. The Mohs hardness scale is a relative ranking of common minerals and their hardness. Gemstones are generally high up on the ranking, as it is important for them to not be easily scratched. In order from soft to hard, the Mohs scale (developed in the early 1800s) is defined by the following index minerals: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), corundum (9), and diamond (10). Half increments are often used, as in the case of beryl that has a hardness of ~7.5–8. Because hardness is a function of bonding within a mineral, it is also technically a property that may vary depending on the direction of scratching. For example, kyanite shows a hardness of 5 parallel to its length and 7 across the length, while garnet exhibits a hardness of 7.5 in all directions. Hardness can also be measured by other methods and scales, such as Vicker’s Hardness or the use of a sclerometer, an instrument that measures the width of a scratch made by a diamond on the sample under controlled conditions.

Photo depicts a crystal of corundum shows rhombohedral parting patterns and underlying irregular fractures.

      Photo by D. Turner.

Photo depicts a cluster of bladed kyanite crystals shows brittle tenacity and splintery parting, yellowish-grey to blue coloration, and would exhibit lower hardness along the length of the crystals than across.

      Photo by D. Turner.

      Fluorescence is a consistent property of some minerals while in others it only occurs when certain impurities are present. Fluorescence is a phenomenon where light with greater energy (and shorter wavelength) excites electrons within a material and upon deexcitation (or relaxation) of the electron to ground state, a photon of lesser energy (and longer wavelength) is emitted. It is a type of luminescence. This is normally tested using ultraviolet light and observed in the visible range with the human eye; however, the process can be observed across a range of activating and fluorescent wavelengths. Fluorite is a common fluorescent mineral and some diamonds can be strongly fluorescent, yet neither of these minerals will always display fluorescence. Other types of luminescence include phosphorescence, thermoluminescence, triboluminescence, and cathodoluminescence.

      1 Ball, S. H. (1935). A historical study of precious stone valuations and prices. Economic Geology, 30(5), 630–642.

      2 Haxel, G. B., Hedrick, J. B., Orris, G. J., Stauffer, P. H., & Hendley II, J. W. (2002). Rare earth elements: Critical resources for high technology. Fact sheet No. 087‐02. United States Geological Survey.

      3 Hazen, R. M., Grew, E. S., Downs, R. T., Golden, J., & Hystad, G. (2015). Mineral ecology: Chance and necessity in the mineral diversity of terrestrial planets. The Canadian Mineralogist, 53(2), 295–324.

      4 Mason, B. & Moore, C. (1982). Principles of Geochemistry. New York: John Wiley & Sons.

      2.1 Earth System Science

      The significance of these components varies for the creation and preservation of different precious materials but all aspects tend to be tied together in one way or another. Diamonds, for example, predominantly form deep within the Earth in a region called the Upper Mantle, where very high pressures and temperatures exist. However, other processes, such as volcanism, are required to bring these diamonds through the mantle and crust to the surface. Natural processes on the Earth’s surface, such as glaciation, can move the diamonds away from their original source and leave a trail of ground kimberlite rock leading back to where the original deposit resides. Alternatively, if enough diamonds were moved by natural processes (e.g., river transport) from their primary geological location to a new secondary location, a diamond deposit could be formed far away from the original source rock. Even in this very limited example, the complexity and interconnectedness of the Earth system is obvious.

       Crust. The Earth’s crust is the uppermost layer. It represents


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